Radio signals in mobile radio are subject to multipath propagation, that is to say a number of versions of the received signal appear in the receiver as a result of reflection, scatter and diffraction of the transmitted radio signal on various obstructions in the propagation path, which versions of the received signal are shifted in time with respect to one another and are subject to different attenuations. The principle of operation of a rake receiver is based on separate evaluation of the received signal versions with the highest energy, which are then superimposed with the correct timing. The expression “rake” in this case figuratively describes the structure of a receiver such as this, with the tines of the rake representing the rake fingers, and the handle of the rake representing the superimposed received signal produced on the output side.
Rake receivers are known and are frequently used in mobile radio stations.
Each rake finger operates with the aid of a set of operating parameters, which determine the function of that rake finger. The set of operating parameters includes the delay time which the rake finger “sets” for the desired propagation path, that is to say it selects one of the versions of the received signal. In the case of CDMA (Code Division Multiple Access) systems, further parameters are added in order to despread the received spread-coded received signal versions in each rake finger. Details relating to the scrambling sequence and the spread code to be despread as well as details relating to the spreading factor on which it is based are signaled in parametric form to the rake finger.
The parameter set is normally stored in a parameter memory, which is accessed by the rake finger.
During operation, the rake fingers which are physically present are generally not all allocated. If an active rake finger with a parameter set which is no longer required is intended to be deactivated and a rake finger with a new parameter set is required, one of the free rake fingers with the new parameter set is activated in good time at the time of the start of a time slot or frame, according to the previous procedure. The change from the rake finger with the parameter set which is no longer required to the rake finger with the new parameter set then takes place by switching from the output of the “old” rake finger to the output of the “new” rake finger.
One disadvantage of this procedure is that, at least temporarily, at least two rake fingers must always be set up in order to change the parameter sets. This results in increased power consumption and, furthermore, the necessity to always keep free (unallocated) rake fingers available for a change.